This invention relates generally to mechanical refrigeration machines such as centrifugal refrigeration machines, and more particularly to an automatic control for such machines.
Mechanical refrigeration machines include, generally, an evaporator or cooler section, a compressor, and a condenser. Often, a refrigerant is circulated through the evaporator, a chilled water coil is positioned in the evaporator, and water is circulated through that coil so that heat is transferred from the water to the refrigerant in the evaporator. The chilled water can then be circulated to a remote location to satisfy a refrigeration load. The refrigerant evaporates as it absorbs heat from the water, and the compressor is arranged to extract refrigerant vapor from the evaporator, compress the refrigerant thereby increasing its temperature, and discharge the refrigerant into the condenser. The refrigerant is condensed and cooled in the condenser and then redelivered to the evaporator where the cycle begins again.
In order to minimize operational cost, it is generally desirable to match the amount of work done by the compressor to that which is needed to satisfy the refrigeration load placed on the refrigeration machine. Commonly, in centrifugal refrigeration machines, this is done by regulating the amount of refrigerant vapor flowing through the compressor by means of a plurality of guide vanes which are positioned between the compressor and the evaporator and which modulate between a fully open and a fully closed position in response to the temperature of the chilled water as it leaves the chilled water coil in the evaporator. When the temperature of this water falls, indicating a reduction in the refrigeration load on the machine, the guide vanes move toward the closed position, decreasing the amount of vapor flowing through the compressor. This decreases the amount of work that must be done by the compressor, thereby decreasing the amount of energy needed to operate the machine. At the same time, this has the effect of increasing the temperature of the chilled water leaving the evaporator. In contrast, when the temperature of the leaving chilled water rises, indicating an increase in the load on the machine, the guide vanes move toward the fully open position. This increases the amount of vapor flowing through the compressor. The compressor does more work, decreasing the temperature of the chilled water leaving the evaporator and allowing the refrigeration machine to respond to the increased refrigeration load.
As a general rule, the compressor operates to maintain the temperature of the chilled water leaving the evaporator at, or within a certain range of, a specific, or setpoint, temperature. This certain range is referred to as the throttling range of the system, and it lends stability to the control system of the machine. A detailed discussion of the throttling range is given in U.S. Pat. No. 3,250,084, granted to Carl M. Anderson on May 10, 1966, for "Control Systems."
It is known that when the compressor and, hence, the refrigeration machine are operating at less than maximum capacity, that is, at partial load conditions, significant energy savings can be achieved without affecting the ability of the machine to satisfy the refrigeration load placed thereon by raising the setpoint temperature. The energy savings are due to the fact that raising the setpoint temperature reduces the amount of work which must be done by the compressor to maintain the chilled water at, or within the throttling range of, the setpoint temperature. The refrigeration load can be satisfied because, provided that the setpoint temperature is not raised too high, the chilled water is still sufficiently cool to absorb enough heat from the refrigeration load to produce the desired effect.
In the past, typically, automatic control of the chilled water setpoint temperature has been accomplished by regulating the setpoint temperature in response to any one or more of several ambient factors. This has often required a complex and costly pneumatic or electronic control system.